Abstract : The objective of this research was to study nonradiative processes occurring in tunable solid state laser crystals, focusing on the first Cr(4+) tunable laser crystal, Cr:Mg2SiO4 developed by our group. Nonradiative processes were investigated in terms of local (ion) - phonon (lattice) mode coupling. A restricted number of phonon modes was shown to be directly involved in the nonradiative deexcitation of Cr4+. Up converted hot luminescence techniques identify the phonon modes involved in the nonradiative decay, and time resolve Raman scattering allowed for the direct measurement of the dynamics of Raman active phonons involved in the nonradiative relaxation. Resonance Raman scattering identified Cr(4+) local modes. This research project has shown that one of the main criteria leading to local phonon mode coupling which triggers nonradiative processes is energy resonance between local and phonon modes. A minor research effort focused on developing new Cr(4+) doped solid state laser materials. This work has led to growing and characterizing new materials with the potential of lasing in the 1.2-1.6 micrometer near infrared spectral region.